The invention relates to a plasma generation and confinement system of a toroidal types, and more particularly to a method and apparatus for providing toroidally segmented passive conductors for stabilizing vertical motion of the plasma in a large toroidal vacuum chamber lined with modules which are segmented in the toroidal direction. FIGS. 1a and 1b, which illustrate the present invention in a representative environment, show cross-sectional views of a tokamak device with only major components. The motion of the plasma that requires stabilization is referred to as being vertical because, for small displacement of the plasma along the vertical axis, the plasma is unstable and will move further from the center of the device in a vertical direction when the device is oriented as shown in FIG. 1a.
Tokamak systems for plasma generation, confinement and control generally have a dual-function stabilization system, namely a passive inner layer of electrical conductors located close to the plasma surface and an active outer layer of coils. Illustrative of such plasma generation systems is the toroidal pinch plasma confinement system of the tokamak configuration shown in U.S. Pat. No. 4,330,864 titled DOUBLE LAYER FIELD SHAPING SYSTEMS FOR TOROIDAL PLASMAS. In that system, a large toroidal vacuum chamber is made of conductive material providing a path for induced current to flow in a horizontal toroidal direction in response to any vertical motion of the plasma. The induced toroidal currents in the wall tend to provide passive stabilization to vertical displacements of the highly elongated plasma in the chamber. Actively controlled coils provide the appropriate poloidal fields required to shape the plasma cross section, position the plasma in the vacuum vessel chamber, and provide magnetohydrodynamic (MHD) stability of the plasma in the chamber. Also provided around the chamber are toroidal field coils which establish an azimuthal magnetic field for stable plasma confinement.
Tokamak experiments show increased performance with increased plasma current. (J. C. DeBoo, et al., "Doublet III Operating Regimes with Improved Energy Confinement," Nuclear Fusion, Vol. 26, No. 2 (1986) p. 211). In the next-generation tokamak reactor devices, increased plasma current is being proposed to provide the plasma confinement required to reach ignition. (N.S. Uckan, "Relative Merits of the Size Field & Current on Ignited Tokamak Performance," Fusion Technology, Vol. 14, September 1988, p. 299). Plasma shaping, especially plasma cross section elongation, is viewed as one of the best means for obtaining higher plasma current. Generally plasma generation and confinement systems of the toroidal type are adapted to provide an elongated plasma. However, the vertical stability properties of a plasma degrade with elongation, and this greatly increases the requirements on the passive and active elements used to stabilize this motion. Reactor geometry also restricts the ability to place passive elements near the plasma, and hence, the passive stability characteristics of highly elongated plasmas are an important consideration in the design of next generation tokamak reactor systems.
In a study by the present inventor, the passive vertical stability properties of highly elongated plasmas are investigated and an assessment is presented on the impact of this passive stability on the next generation tokamaks. (J. A. Leuer, "Passive Vertical Stability In The Next Generation Tokamaks," Fusion Technology, Vol. 15, No. 2, Part 2A, pp. 489-494, March 1989). In particular, an early variant of an ITER (International Thermonuclear Experimental Reactor) configuration proposed by the United States is used as a baseline to evaluate the impact of plasma elongation on its plasma current carrying capability and on its vertical stability properties. That published study presents a methodology which predicts the stability characteristics of a plasma equilibrium moving vertically within a set of toroidally continuous resistive elements. Results were presented which relate plasma elongation and passive element properties to stability margin and vertical growth rate.